1. Field of the Invention
The present invention relates to ball screws and more particularly, to a double nut ball screw capable of sensing preload.
2. Description of the Related Art
A conventional double nut ball screw generally comprises two nuts threaded onto a screw rod, a preloading plate mounted between the two nuts, and a connection key having two opposite ends respectively mounted in two keyways provided on peripheries of the two nuts. Thus, the two nuts maintain a preload to hold down the preloading plate during rotation of the screw rod, preventing occurrence of an axial gap during movement of the two nuts and enhancing the rigidity, positioning accuracy and position stability of the ball screw in the operation.
After a long use of the ball screw, the balls, the screw rod and the nuts may start to wear, causing occurrence of an axial gap. When an axial gap occurs, the preload between the two nuts will be relatively reduced, lowering the rigidity, positioning accuracy and position stability of the ball screw.
It can be seen from the above, the preload is one of the key indicators for the measurement of the rigidity, positioning accuracy and positioning stability of the ball screw.
Taiwan Patent Publication No. 201422946 discloses a ball screw that allows real-time monitoring of the preload, which has a force sensor set between the two nuts of the ball screw for monitoring the preload between the two nuts.
Further, Japan Patent JPH05-141498 discloses a technique of setting a preload sensing means between the two nuts of a ball screw to detect the preload.
However, the above-described prior art techniques simply have a force sensor be set between the two nuts of a ball screw without providing any measure to enhance the preload sensing sensitivity and effect.